Seat changed even though I did online check in.

I was flying to East London this morning the 27th February 2023. Flight FA388 I checked in and got seat 10F which was the window seat. When I came to drop my bag off I saw the ticket issued has my seat number changed to 17E which is the middle seat. When the staff was asked I was told that someone had bought the seat I chose and there was nothing they could do to assist me. I’m a frequent flyer and I’m honestly disappointed with the service. I mean what’s the point of doing online check in if you’re just going to be allocated another seat when you get on the plane.

Definitely an unhappy customer. Expected way more.